Oil & Gas Geology ›› 2018, Vol. 39 ›› Issue (4): 664-675.doi: 10.11743/ogg20180404

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Abnormal pressure system and its origin in the Nanpu Sag,Bohai Bay Basin

Zhang Lei1,2,3, Xiang Caifu1,2, Dong Yuexia4, Zhang Mengyuan1,2,5, Lyu Yue1,2,6, Zhao Zhongxin4, Long Huashan4, Chen Shuang1,2   

  1. 1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum(Beijing), Beijing 102249, China;
    2. College of Geosciences, China University of Petroleum(Beijing), Beijing 102249, China;
    3. Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation Mechanism, Ministry of Education, China University of Geosciences(Beijing), Beijing 100083, China;
    4. Jidong Oilfield Company Ltd., Tangshan, Hebei 063000, China;
    5. Langfang Branch, PetroChina Research Institute of Petroleum Exploration and Development, Langfang, Hebei 065007, China;
    6. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
  • Received:2017-03-14 Revised:2018-04-02 Online:2018-08-28 Published:2018-07-23

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Abstract: Abnormal pressure in the Nanpu Sag is prevalent,but its distribution in varying tectonic zones and its genetic mechanisms are rarely studied.So we depicted the architectural features of the pressure for varying tectonic zones in detail based on the data from 1 354 drill stem tests(DST),repeated formation pressure tests(RFT),and the well logging curves of 357 wells.The results show that vertically,the formation pressure system can be classified into three zones in the Nanpu sag,namely,the upper normal pressure zone(with burial depth less than 1 800 m),the middle transitional zone(with burial depth ranging from 1800m to 2 400 m),and the lower abnormal high pressure zone(with burial depth more than 2 400 m).Large part of the Shahejie Member 3 is controlled by significant abnormal overpressure with pressure coefficient up to 1.9,and the depth from ground to the top of the overpressure zone is about 2 400 m.The middle and upper layers of the Laoyemiao Formation overpressure is less intense with pressure coefficient about 1.2.However,the Dongying Member 1 and 2 offshore,develops abnormal low pressure in some area.In order to further discuss the abnormal pressure mechanisms in the Nanpu Sag,we resorted to methods such as numerical simulation,vertical effective stress-acoustic time cross-plot,and comprehensive analysis of hydrocarbon generation and expulsion for source rocks.Consequently,we propose four conclusions:(1)the large-scale overpressure in Es3 mainly originates from the sandstone disequilibrium compaction during the Dongying period,and contribution from hydrocarbon generation is relatively minor;(2)in Minghuazhen period,hydrocarbon generation becomes the main mechanism for generating pressure,while the influence from disequilibrium compaction is rather limited;(3)the lower-degree overpressure of the middle and upper layers is thought to be induced by the vertical fluid migration from the deep overpressured formations through active fault zone;(4)the regional uplifting and denudation during the late Dongying period result in pore rebound of rock matrix and fluid shrinkage,which are considered to be the main cause of the abnormal low pressure in the Nanpu sag.

Key words: disequilibrium compaction, hydrocarbon generation, overpressure transfer, hydrothermal effect, abnormal high pressure, Nanpu Sag, Bohai Bay Basin

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